The specific sequence of biochemical events leading to irreversible injury and subsequent death of ischemic cells is not known. Recent investigations have suggested that the changes in ionized calcium levels in serum may incite a number of adverse reactions in shock. In addition, Ca2+ shifts between extra- and intracellular spaces have been linked to several abnormalities in cellular metabolism. It has been postulated that the intracellular Ca2+ concentration is a crucial mediator of cell injury, and that the increased influx of extracellular Ca2+ into cells (associated with cellular loss of Mg2+) is a final common pathway in cell death. In the proposed studies, two approaches will be used to test the above hypothesis in rats during hemorrhagic shock and following resuscitation. 1) Changes in serum [Ma2+] ionized [Ca2+] and total [Ca] will be related to pH and albumin levels and to the concentrations of these cations in red blood cells. The effects of the above alterations on RBC volume, osmotic fragility, permeability to Ca2+ and ATP content will be assessed. In addition, the affinity of various Ca2+ entry blockers (CEB's) for their binding sites on normal RBC membranes will be determined and correlated with their beneficial effects on cell homeostatis during resuscitation. 2) In vivo measurements of the resting membrane potential will be utilized to evaluate the changes in skeletal muscle and liver cell [Ca2+] and [Mg2+] and to relate them to measured serum PTH levels. The use of calcium supplementation, calcium-free intravenous fluids and calcium entry blockers have all been suggested as adjunctive measures in the treatment of hypovolemic shock. The results remain contradictory since the distribution of calcium ions across cell membranes in hemorrhagic shock and the composition of the ideal resuscitation fluid with respect to Ca2+ is not known. The proposed project is expected to establish the necessary information on Ca2+ and Mg2+ changes during hemorrhagic shock in addition to assessing the possible beneficial effects of CEB's in resuscitation. The data will provide the necessary basis for evaluation of the proposed role of several Ca2+- activated endogenous substances (i.e. endorphins, eicosanoids and oxygen free radicals) in the complex response to profound hemorrhage and the possible use of their respective blockers in resuscitation.